Urban turbulent kinetic energy dissipation rates as influenced by nocturnal low-level jets

Nocturnal low-level jets (LLJs) are responsible for mesoscale transport and dispersion, for example advecting large quantities of moisture into the U.S. Great Plains during summer. LLJs are also assumed to affect smaller-scale transport and dispersion processes. The present study quantifies the mechanisms by which mesoscale phenomena, like nocturnal LLJs, affect turbulence and transport within urban areas.

The study utilizes observations collected during the Joint URBAN 2003 field experiment, which documented atmospheric transport and dispersion in Oklahoma City, Oklahoma, during July 2003. Over one hundred three-dimensional sonic anemometers were deployed in and around the urban area to monitor wind speed, direction, and turbulence during dispersion experiments in the city center. Sonic anemometers were sited in various urban microenvironments, such as within a street canyon, at an intersection of two boulevards, atop a building in the built-up city center, in a less-built-up area just downwind of the city center, etc. The inertial dissipation method has been applied to 30-minute averaged spectra from some of these sonic anemometers to calculate turbulence kinetic energy (TKE) dissipation rates. We find that, although daytime values of dissipation rates are typically at least a factor of two larger than nighttime values of dissipation, some significant nocturnal events include short “bursts” with dissipation rates on the order of daytime values.

The timing of these high-dissipation events has been compared with an hourly climatology of nocturnal low-level jet (LLJ) behavior in the OKC region based on boundary-layer wind profiler data. Many of these events are correlated with strong nocturnal low-level jets, suggesting that episodic turbulence bursting events induced by low-level jets descend to the levels observed with sonic anemometers, inducing enhanced TKE production and dissipation rates.